1. Field of the Invention
The present invention relates generally to an HSDPA (High Speed Downlink Packet Access) communication system, and in particular, to an apparatus and method for transmitting TBS (Transport Block Set) size information for user data.
2. Description of the Related Art
In general, HSDPA refers to a scheme for transmitting data using HS-DSCH (High Speed-Downlink Shared Channel), a downlink data channel for supporting high-speed downlink packet transmission, and its associated control channels in a UMTS (Universal Mobile Telecommunication System) communication system. AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Selection) schemes have been proposed in order to support the HSDPA. The AMC, HARQ and FCS schemes will be described herein below.
First, a description of the AMC will be made. The AMC is a data transmission scheme for adaptively determining a modulation scheme and a coding scheme of a data channel according to a channel condition between a specific Node B and a UE (User Equipment), thus to increase the overall utilization efficiency of the Node B. Therefore, the AMC supports a plurality of modulation schemes and coding schemes, and modulates and codes a data channel signal by combining the modulation schemes and the coding schemes. Commonly, each combination of the modulation schemes and the coding schemes is called “MCS (Modulation and Coding Scheme),” and there are defined a plurality of MCS levels of #1 to #n according to the number of the MCSs. That is, the AMC adaptively determines an MCS level according to a channel condition of a UE and a Node B to which the UE is wirelessly connected, thereby increasing the entire utilization efficiency of the Node B.
Next, a description will be made of the FCS. The FCS is a scheme for fast selecting a cell having the best channel condition among a plurality of cells, when a UE supporting the HSDPA (hereinafter referred to as “HSDPA UE”) is located in a cell overlapping region, or a soft handover region. Specifically, in the FCS, if an HSDPA UE enters a cell overlapping region between a first Node B and a second Node B, the HSDPA UE establishes radio links to a plurality of cells, i.e., the first Node B and the second Node B. Here, a set of the cells to which the HSDPA UE has established the radio links is called an “active set.” The UE reduces overall interference by receiving HSDPA packet data only from the cell maintaining the best channel condition among the cells included in the active set. Here, a cell in the active set, which transmits HSDPA packet data due to its best channel condition, is called a “best cell,” and the HSDPA UE periodically checks channel conditions of the cells in the active set and transmits a best cell indicator to the cells belonging to the active set in order to replace the current best cell with a new best cell having the better channel condition. The best cell indicator includes a cell ID of a cell selected as a best cell, and the cells in the active set receive the best cell indicator and detect the cell ID included in the best cell indicator. Each of the cells in the active set determines whether the received best cell indicator includes its own cell ID. As a result of the determination, if the best cell indicator includes its own cell ID, the corresponding cell transmits packet data to the HSDPA UE over HS_DSCH.
Finally, a description will be made of the HARQ, especially n-channel SAW HARQ (Stop and Wait Hybrid Automatic Retransmission Request). The HARQ newly proposes the following two plans in order to increase transmission efficiency of the existing ARQ (Automatic Retransmission Request). First, a retransmission request and a response are exchanged between a UE and a Node B. Second, defective data is temporarily stored, and combined with retransmitted data of the corresponding data. Further, the HSDPA has introduced the n-channel SAW HARQ in order to make up for a shortcoming of the conventional SAW ARQ. The SAW ARQ does not transmit the next packet data until it receives ACK for the previous packet data. Therefore, in some cases, the SAW ARQ must await ACK, although it can currently transmit the next packet data. However, in the n-channel SAW HARQ, the next packet data is continuously transmitted before ACK for the previous packet data is received, thereby increasing utilization efficiency of channels. That is, if n logical channels are established between a UE and a Node B, and the n logical channels can be identified by time and unique channel numbers, then the UE can recognize a channel over which packet data was received, and rearrange the received packets in the right reception order, or soft-combine the received packets.
In a communication system supporting the HSDPA (hereinafter, referred to as HSDPA communication system) which increases communication efficiency by supporting AMC, FCS and HARQ, a plurality of UEs share some of downlink transmission resources. The downlink transmission resources include transmission power and OVSF (Orthogonal Variable Spreading Factor) codes (or orthogonal codes). The HSDPA communication system uses 10 OVSF codes for SF (Spreading Factor=16, and 20 OVSF codes for SF=32.
In the HSDPA communication system, a plurality of UEs can simultaneously use a plurality of available OVSF codes at a specific time. That is, in the HSDPA communication system, it is possible to enable OVSF code multiplexing among a plurality of UEs at a specific time. The OVSF code multiplexing will be described with reference to FIG. 1.
FIG. 1 illustrates an exemplary method of assigning OVSF codes in a general HSDPA communication system. A description of FIG. 1 will be made for SF=16.
Referring to FIG. 1, OVSF codes are defines as C(i,j) according to the positions of a code tree. In an OVSF code C(i,j), ‘i’ denotes the SF value and ‘j’ denotes the order of the corresponding OVSF code from the leftmost side in the OVSF code tree. For example, C(16,0) indicates an OVSF code with SF=16 located in the first place from the leftmost side in the OVSF code tree. In FIG. 1, for example, 10 OVSF codes of a 7th OVSF code C(16,6) to a 16th OVSF code C(16,15) are assigned to the HSDPA communication system. The 10 OVSF codes can be multiplexed to a plurality of UEs, as illustrated in Table 1.
TABLE 1TimeUsert0t1t2AC(16,6)~C(16,7)C(16,6)~C(16,8) C(16,6)~C(16,10)B C(16,8)~C(16,10) C(16,9)~C(16,10)C(16,11)~C(16,14)CC(16,11)~C(16,15)C(16,11)~C(16,15)C(16,15)
In Table 1, A, B and C denote users (or UEs), which are using the HSDPA communication system. As illustrated in Table 1, the users A, B and C perform code multiplexing on the OVSF codes assigned to the HSDPA communication system at timing points t0, t1 and t2. The number of OVSF codes to be assigned to the UEs and their positions on the OVSF code tree are determined by the Node B, and the Node B determines the number of OVSF codes and their positions taking into consideration an amount of user data for each UE stored in the Node B.
The HSDPA communication system proposes that such control information as the OVSF code information should be transmitted to UEs over downlink control channels. For better understanding, reference will be made to a channel structure for the HSDPA communication system.
The HSDPA communication system includes high-speed downlink shared channels (HS-DSCH), downlink control channels and uplink control channels. The high-speed downlink shared channel transmits user data to UEs using OVSF codes assigned to the HSDPA communication system. In order to support the AMC, HARQ and FCS schemes newly introduced to support the HSDPA communication system, it is necessary to exchange control information between the Node B and the UE, and the control information is transmitted over the downlink control channel and the uplink control channel. The control information transmitted over the uplink control channel includes (i) channel quality information (CQI) periodically reported to the Node B by the UE, (ii) an ACK (Acknowledgement) signal used by the UE to report whether received user data is defective, and (iii) best cell information used by the UE to report a cell providing the best channel condition by comparing channel conditions of the cells within its coverage.
In addition, control information transmitted over the downlink control channel includes (i) HI (HS-DSCH Indicator) indicating to a UE that user data will be transmitted over a high-speed downlink shared channel, (ii) MCS level information, (iii) TBS (Transport Block Set) size information, (iv) OVSF code information to be assigned to the corresponding UE, (v) HARQ information, and (vi) CRC (Cyclic Redundancy Check) information. Of the control information transmitted over the downlink control channel, the sum of the MCS level information, the TBS size information and the OVSF code information is called “TFRI (Transport Format Resource Information).”
The control information stated above is transmitted over two control channels of an associated DPCH (dedicated physical channel) and a SCCH (shared control channel). The “associated dedicated physical channel” means a dedicated physical channel established between a UE and a Node B, both supporting the HSDPA communication, on a one-to-one basis, and the dedicated physical channel transmits the HI. The HI indicates whether HSDPA service data will be transmitted to a UE over a high-speed downlink shared channel in the near future. If the HSDPA service data is transmitted to the UE, the HI indicates a shared control channel over which the UE should receive the concerned control information, among a plurality of shared control channels used in the HSDPA communication system. For example, in the case where 4 shared control channels exist in the HSDPA communication system, if the 4 shared control channels are assigned unique integers 0 to 3 and the HI is comprised of 2 bits, then (1) non-transmission of the HI means that there exists no HSDPA service data to be transmitted to the corresponding UE, (2) HI=0(00) indicates that control information for the HSDPA service data should be received over a shared control channel #0, (3) HI=1(01) indicates that control information for the HSDPA service data should be received over a shared control channel #1, (4) HI=2(10) indicates that control information for the HSDPA service data should be received over a shared control channel #2, and (5) HI=3(11) indicates that control information for the HSDPA service data should be received over a shared control channel #3,
The shared control channel transmits the remaining control information except the HI, and a structure of the shared control channel will be described with reference to FIG. 2.
FIG. 2 illustrates a structure of a shared control channel in a common HSDPA communication system. Referring to FIG. 2, the shared control channel has a 2 ms period comprised of 3 slots. The reason that the shared control channel transmits a signal at a period of 2 ms is because a data transmission unit over the high-speed downlink shared channel is 3 slots. For example, the ongoing standardization session proposes that one of the 3 slots which become the data transmission unit of the high-speed downlink shared channel transmits the HARQ information, and the remaining 2 slots transmit the TFRI and the CRC, respectively. If the UE detects an HI field filled with information while continuously monitoring the HI field on an associated dedicated physical channel established between the UE and the Node B, the UE reads information on a corresponding shared control channel designated by the HI information and receives a high-speed downlink shared channel corresponding to the information read from the corresponding shared control channel.
In the HSDPA communication system, information needed to properly process data received by a physical layer of the UE includes TB (Transport Block) size information, TBS size information, channel coding information, modulation information, rate matching information, and code information. On the information stated above, the channel coding information and the modulation information are transmitted from the Node B to the UE through MCS level information, while the code information is transmitted from the Node B to the UE through OVSF code information. In addition, a size of the transport block is determined during initial call setup, and the size of the transport block remains unchanged (i.e., fixed size) while the call is maintained, so it is not necessary to separately transmit information on the size of the transport block from the Node B to the UE.
Further, the TBS size information indicates the number of transport blocks transmitted for a single TTI (Transmission Time Interval), and the rate matching information indicates a repetition or puncturing technique performed on user data by a physical layer of the Node B performs repetition or puncturing. The TBS size information is transmitted over the TFRI field illustrated in FIG. 2, and the rate matching information is not transmitted separately, because the rate matching technique is determined depending on the TBS size.
Next, a structure of a physical layer for a transmitter in the HSDPA communication system will be described with reference to FIG. 3.
FIG. 3 illustrates a channel structure of a physical layer for a transmitter in a common HSDPA communication system. Referring to FIG. 3, transport blocks to be transmitted are transmitted from an upper layer to a physical layer, i.e., over a transport channel. The transport blocks transmitted from the upper layer undergo concatenation or segmentation according to their size. For example, in FIG. 3, the transport blocks transmitted from the upper layer undergo concatenation (Step 301). Here, the transport blocks are transmitted from the upper layer to the physical layer for each TTI. The transport blocks transmitted in the TTI unit constitute a transport block set, and the number of transport blocks transmitted over the transport block set becomes a size of the transport block set. Header information is attached to the transport blocks, or the transport block set transmitted from the upper layer (Header Attachment) (Step 302). The header information may include such information as serial numbers that can be used for sequentially arranging the transport blocks in the transport block set at a receiver corresponding to the transmitter. CRC is attached to the header information-attached transport block set (Step 303). Here, for the CRC, a 24-bit CRC operation can be considered.
The CRC-attached transport block set is segmented into code blocks with a size proper for channel coding for error correcting codes (Step 304), and then subject to channel coding for channel transmission (Step 305). Here, the channel-coded data is called a “coded block.” After the code block segmentation, i.e., at a point D4, information bits constituting the transport blocks are converted into a symbol at a point D5 through the channel coding. The coded block undergoes rate matching taking into consideration a length of a physical layer frame and a spreading factor in order to actually transmit the coded block to the physical layer (Step 306). That is, the rate matching is a process of matching a size of the coded block to an amount of information that can be actually transmitted over the physical channel. For example, if the number of symbols output through the channel coding is D5 and the number of symbols finally transmitted over the physical channel is D9, then the number of symbols after the rate matching is matched to D9. That is, for the rate matching, puncturing is performed for D5>D9 and repetition is performed for D9>D5, thus to match the number of symbols at a point D5 to the number of symbols at a point D9.
The rate-matched data is segmented in a unit that can be transmitted over a physical channel (Physical Channel Segmentation) (Step 307). The physical channel segmentation is performed to segment the whole data in a size proper for each code, since a high-speed downlink shared channel can be comprised of a plurality of codes. The physical channel-segmented data is interleaved in order to prevent a burst error (Step 308), and the interleaved data is finally mapped to a physical channel and then transmitted over the physical channel (Physical Channel Mapping) (Step 309).
An amount of user data to be transmitted is changed as follows, as the user data passes through the processes illustrated in FIG. 3.
D1=TB_Size (size of transport block)*TBS13 Size (size of transport block set)
D2=D1+Header_Size (size of header)
D3=D2+CRC (e.g., 24 bits)
D4=D3
D5=D4*1/CR (where CR denotes a coding rate)
D6=D5+RM (size of rate matching)
D7=D6
D8=D7
D9=D8={(TB_Size*TBS_Size+Header_Size+CRC)/CR+RM}
Further, in FIG. 3, a transmission unit of the user data is changed as follows, as the user data passes through the processes illustrated in FIG. 3. The transmission unit becomes an IB (information bit) unit at D1 to D4, a symbol unit at D5 to D8, and a MS (modulated symbol) unit at D9. That is, the information bits are converted to a symbol through channel coding, and the symbol is converted to a modulated symbol through modulation.
Since the D9 means the total sum of data actually transmitted over the physical channel, it can be expressed asD9=NC*Code_capa=NC*[((chip rate per time slot)/SF)*(number of time slots per TTI)*MO)]=NC*MO*480 (3 time slots)*2560 (chip rate per time slot)/16(SF)  Equation (1)
In Equation (1), NC denotes the number of codes, Code_capa denotes an amount of data that can be transmitted by one code, SF denotes a spreading factor, and MO denotes a modulation order. Further, in Equation (1), a unit of the data amount becomes a symbol unit, Equation (1) can be rewritten as Equation (2). Here, it is assumed that SF=16.[TB_Size*TBS+Header_Size+CRC]/CR+RM=NC*480*MO  Equation (2)
Further, Equation (2) can be written asRM=NC*480*MO−[TB_Size*TBS_Header_Size−CRC]/CR  Equation (3)
In Equation (3), if repetition is performed for rate matching, the parameter RM becomes a positive number, and if puncturing is performed for rate matching, the parameter RM becomes a negative value.
A data amount in each process of FIG. 3 will be described with reference to FIG. 4.
FIG. 4 illustrates an amount of data in each process in the channel structure of the physical layer of FIG. 3. Before a description of FIG. 4, it should be noted that an amount of data finally transmitted over a physical channel is D9 as described in conjunction with FIG. 3, and the D9 is defined by a Node B at a certain timing point. That is, the D9 is determined based on the number of codes assigned to a given UE at a certain timing point and an MCS level. The transport block size TB_Size, the CRC size and the header size Header_Size are also constants which are not changed while the corresponding call is maintained. However, the transport block set size TBS_Size is a variable which is changed according to an amount of data for the UE, stored in the Node B. In other words, in Equations (1) to (3), parameters which are changed for each TTI include TBS (Transport Block Set), NC (Number of Codes), MO (Modulation Order), and CR (Coding Rate). These parameters are transmitted from the Node B to the UE for each TTI over a TFRI field on the shared control channel.
Referring to FIG. 4, when transport blocks are transmitted from an upper layer, the transport blocks undergo transport block concatenation according to their sizes as illustrated in conjunction with FIG. 3, and an amount of the concatenated transport blocks becomes D1. When header and CRC are attached to the concatenated transport blocks, an amount of the header/CRC-attached transport blocks becomes D3. When the header/CRC-attached information bits undergo code block segmentation and channel coding, an amount of the channel-coded data becomes D5. When D5 symbols are rate matched, an amount of the rate-matched data becomes D6. When D6 symbols are subject to physical channel segmentation, an amount of the segmented data becomes D7. Here, D6 is equal to D7 in a data amount, but rate-matched symbols are segmented according to an amount of the physical channel. In FIG. 4, it is assumed that repetition is performed for the rate matching.
A rate matching process by the physical layer structure of FIG. 3 will be described with reference to FIGS. 5A and 5B.
FIGS. 5A and 5B illustrate a common rate matching technique. Referring to FIGS. 5A and 5B, if a Node B, or a transmitter determines a rate matching technique, then the physical layer channel structure of FIG. 3 repeats or punctures coded blocks represented by D5 at regular intervals according to the rate matching technique, and transmits the rate-matched coded blocks to a UE, or a receiver after channel processing. The receiver then inserts 0's in the punctured portion (0 Insertion) if the rate matching value (or parameter) is a negative value, i.e., if the coded blocks underwent puncturing, in order to match a size of the coded blocks to D5, and then provides the 0-inserted coded blocks to a channel decoder.
In contrast, if the rate matching value is a positive number, i.e., if the coded blocks underwent repetition, the receiver sums up the repeated bits in order to match a size of the coded blocks to D5, and provides the rate-matched coded blocks to the channel decoder. That is, the receiver secures correct channel decoding, when it recognizes a rate matching value transmitted by the transmitter. Further, in the HSDPA communication system, information on the transport block set (TBS), the number of codes (NC) and the coding rate (CR) is reported from a Node B to a UE for each TTI, thus to enable the Node B and the UE to calculate the same rate matching value. Although the UE can correctly determine the number (or TBS size) of transport blocks transmitted from the Node B by calculating the rate matching value, the Node B transmits to the UE the TBS size information for each TTI, i.e., transmits the downlink control information unnecessarily, causing an unnecessary waste of downlink channel resources.